The present invention is directed to coating compositions based on polyisocyanates and aldimines which have improved pot lives without a corresponding increase in the dry times of the resulting coatings under ambient conditions.
Coating compositions which may be cured at room temperature are known. One-component coating compositions contain fully reacted polyurethanes as the binder. These compositions have the advantage that they are available as fully formulated systems which may be directly applied to suitable substrates without any preliminary steps except for mild stirring. Disadvantages of these systems are that large amounts of organic solvents are needed to reduce the viscosity of fully reacted, i.e., high molecular weight, polyurethanes. The coating compositions are cured by evaporation of the solvent which is objectionable from an environmental perspective. In addition, in order to solubilize the polyurethanes in organic solvents, they must be essentially linear polyurethanes. While such polyurethanes possess properties which are suitable for many applications, they do not provide certain properties, e.g., solvent resistance which may be obtained from crosslinked polyurethanes.
Two-component coating compositions are also known. These compositions come in two containers. The first contains a polyisocyanate, while the second contains an isocyanate-reactive component, generally a polyol. The components are not mixed until they are ready to be used. One advantage of these compositions is that because the components are not pre-reacted to form a high molecular weight polymer, a suitable processing viscosity can be achieved without the need for large amounts of organic solvents. In addition, higher functional components can be used to obtain highly crosslinked coatings which possess properties which surpass those possessed by one-component coatings.
The disadvantages of these compositions is that they cannot be applied without a preliminary mixing step in which it is critical that the components are mixed in the right proportions. In addition, special metering and mixing equipment is needed to conduct this process on a commercial scale. If the components are mixed in the wrong proportions, then the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they must be used in a timely fashion. If not, they continue to react until an unusable solid is obtained.
Coating compositions which possess the advantages of the known one- and two-component coating compositions without possessing their disadvantages have been disclosed in copending applications, U.S. Ser. Nos. 08/171,281 and 08/171,550. Even though coatings prepared in accordance with these copending applications possess many desirable properties, further improvements are needed in the pot lives of the compositions. In particular, the viscosity of these compositions increases too rapidly prior to being applied to a substrate and cured.
Accordingly, it is an object of the present invention to provide increased pot lives without significantly increasing the dry times of the resulting coatings and without altering any of the other desirable properties of the compositions.
This object may be achieved with the coating compositions of the present invention which contain polyisocyanates and aldimines and also the water-adsorbing zeolites described hereinafter to increase the pot life. It is surprising that an increase in the pot life can be obtained by incorporating these zeolites because contrary to the prior art, the Applicants have discovered that water does not hydrolyze aldimines to the corresponding amine.
In addition, in conventional pigmented, one-component, moisture-curing coating compositions containing polyisocyanates, water present in the pigments does affect the pot life. However, the pot life is still adequate, i.e., generally at least 12 to 24 hours, to provide sufficient time to apply the coating compositions before the viscosity increases to the point where it is not possible to apply the coating compositions using conventional spray equipment without adding additional solvent.
To the contrary the examples of the subject application demonstrate that pigmented coating compositions containing polyisocyanates and aldimines, which are essentially blocked amines, increase in viscosity much more rapidly. Since water does not hydrolyze the aldimines to form the corresponding amines (which would then rapidly react with the polyisocyanate component), there is no reason to expect that water is the cause of the more rapid increase in viscosity and shortened pot life. Accordingly, it is surprising that the addition of the water-adsorbing zeolites described hereinafter would have any affect on increasing the pot life of the systems.
For the preceding reasons, it would not be expected that the addition of zeolites would have any effect on increasing the pot life of the composition.
U.S. Pat. Nos. 3,420,800 and 3,567,692 disclose coating compositions containing polyisocyanates and either aldimines or ketimines. However, these patents do not teach the use of zeolites to increase the pot life. In addition, U.S. Pat. No. 5,243,012 teaches that tin compounds may be used to increase the pot life of coating compositions containing polyisocyanates and polyaspartic acid derivatives that contain secondary amino groups. Copending application, U.S. Ser. No. 08/171,304, teaches that tin compounds may be used to increase the pot life of coating compositions containing polyisocyanates and aldimines.